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Ethnic Kurds have been protesting on border towns to Syria in Turkey against the IS assault on Kobane

Fighters from the so-called Islamic state (IS) have reportedly entered the eastern part of the Kurdish town of Kobane.

They have effectively surrounded the town - which sits hard up against the Syrian-Turkish border - from three sides and are firing into it from higher ground.

If Kobane falls it will be a serious set-back for the Kurds. But it will inevitably also raise fundamental questions about the scope and capacity of the US air campaign to combat IS, and about the diverse goals of the anti-IS coalition that the US has assembled.

Kobane, which is a small salient of land poking into Syrian territory, is in no sense an easy place to defend.

Short of a major push across the border by Turkish forces, who would have to seize and hold the high ground beyond the town, even reinforced - but still underarmed - Kurdish fighters might find it hard to hold on.

Kurdish spokesmen of course claim that Turkey is actually hindering such reinforcements crossing the border - highlighting a fundamental ambivalence in the Turkish position, which I will turn to in a moment.

But while Turkey has threatened to establish a buffer zone in Syria and Iraq to push IS back from its own frontier, this would be a major military undertaking and draw the country directly into what could fast become a quagmire.

Many might wonder why US air power has not done more to stem the IS advance. Well for one thing the focus of what you might call US "tactical" strikes has been largely in Iraq. The air strikes in Syria have mainly been against IS command infrastructure and economic targets like oil installations.

Brigadier Ben Barry, the land warfare analyst at the International Institute for Strategic Studies (IISS) in London notes that IS offensives typically begin with significant artillery preparation - several days of barrages from artillery, mortars, tanks and so on.

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Deep-rooted divisions between the Turkish government and Kurds in Turkey have complicated the situation

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Turkish President Erdogan warned on Tuesday that Kobane would fall to the hands of IS militants

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The view of an IS flag from the Turkish side of the border near Kobane

Attacking IS from the air however, is not easy. Their guns may be well camouflaged when not firing, and even rudimentary spotters can identify US aircraft in the vicinity.

Brigadier Barry says that the problems around Kobane illustrate "not so much the lack of a land component but the lack of any real ability to synchronise the air effort with what is taking place on the ground".

"There is," he says, "a powerful argument for having Western special forces on the ground in the forward air control role to help spot and guide aircraft to their targets."

There may also be a problem in that for all the sorties flown, the US and its allies may simply not have sufficient aircraft in theatre to mount the sort of surge in activity that is required to stop IS in its tracks wherever it seeks to advance. More planes and bases closer to the action - airfields in Turkey for example - would help.

But the constant mantra from US spokesmen is that this is a campaign that will take months and years, not days and weeks. Inevitably there are going to be setbacks along the way and the loss of Kobane could represent a serious initial reverse.

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There have been protests across Europe calling for more action for Kobane by the international community

Such a reverse could have profound implications. The coalition to combat IS is broad rather than deep.

Many countries see it as a means of pursuing their own agendas in the region. A common enemy does not really provide a common interest.

This is a region facing profound problems: the fragmentation of existing state structures; fundamental divisions between Shia Iran and the Sunni Arab states; not to mention Turkey's long-standing and complicated relationship with the Kurds.

The conflict close to Turkey's own border at Kobane illustrates the Turkish government's ambivalence.

It is staunchly opposed to the Syrian regime of President Assad and while it sees IS as a threat, it is uneasy about doing anything that might appear to strengthen Mr Assad's grip on power.

Then there are the Kurds. Turkey is equally uneasy about strengthening their military capabilities.

But as IS approaches ever closer to its borders, Turkey may have to decide exactly where its interests lie.

A long campaign against IS requires not just sustained military activity but political cohesion as well.

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Turkish tanks and troops have been lined up on the border but so far they have not crossed into Syria

The early crisis in Kobane does not bode well, highlighting many of the problems with this campaign that have existed from the outset.

But it is early days yet. Reports of stepped up US air activity around Kobane may have some impact. But many Kurds fear this could all be a case of too little too late.